The invention is in the field of electronic reproduction technology and is directed to a method for the electronic assembly of signatures in a raster generator from a plurality of printed pages that are present as high-resolution contone map.
In reproduction technology, printer's copies that contain all elements to be printed such as texts, graphics and images are generated for printed pages. A separate printer's copy that contains all elements that are printed in the respective color is generated for each ink in chromatic printing. For four-color printing, these are the inks cyan, magenta, yellow and black (C, M, Y, K). The printer's copies separated according to inks are also called color separations. The printer's copies are usually screened and exposed on films with high resolution that are then further-processed for the production of printing forms (printing plates, printing cylinders). Alternatively, the printer's copies can also be directly exposed on printing plates in special recorders. For checking the content and the colors of the printed pages, printer's copies are exposed in proof recorders with a recording process that simulates the printing process in a chromatic output.
FIG. 2 shows the work sequence that was previously mainly employed in the prior art in the exposure of printer's copies for printed pages that had been generated in the page description language PostScript. PostScript data 1 are supplied to a raster image processor (RIP) (2) that can be a computer specifically optimized for this job or a program on a standard computer. PostScript data 1 for every color separation are normally generated in a pre-process for every color separation of a printed page and are forwarded to the RIP (2) (separated PostScript). Alternatively, a chromatic printed page can also be generated in a single PostScript dataset (composite PostScript). The case of separated PostScript data 1 shall be explained in greater detail below.
In a first step, the PostScript data 1 are analyzed in an interpreter 3 and resolved into a sequence of simple graphic objects. For that purpose, the printer's copy is divided into horizontal strips (bands) that are successively processed. FIG. 3 shows a band excerpt 9 with a few objects generated by the interpreter. The band excerpt 9 is divided into recording pixels 10. In the example of FIG. 3, the band excerpt is 8 pixels high, numbered from 0 to 7, and 32 pixels wide, numbered from 0 to 31. The resolution can be symmetrical (the same in horizontal and vertical direction) or asymmetrical, for example twice as great horizontally as vertically. The objects A through E (11,12,13,14,15) describe sub-segments of text, graphics or image elements that fall within the band excerpt 9.
The interpreter outputs the objects A through E (11,12,13,14,15) in a data format that is referred to as display list 4 (FIG. 2). For each object, the data format describes its geometrical shape and the gray scale value with which it is filled. The objects A through E (11,12,13,14,15) appear successively in the display list 4 in the sequence in which the corresponding page elements are described in the PostScript data. Objects that appear later in the display list (4) can thereby partly or entirely cover objects that appeared earlier in the display list 4. In the example of FIG. 3, the object A 11 is partly covered by the object B 12. Likewise, the objects D 14 and E 15 cover the object C.
In a further step in the RIP 2, the display list 4 is supplied to a raster generator 5 that successively converts the objects of the display list 4 into surfaces filled with raster points and enters them into a bit map memory 7 as bit map data 6. The raster point size is thereby varied dependent on the gray scale value of the object in the display list 4. The bit map data 6 of objects that appear later in the display list 4 respectively overwrite the corresponding areas of the bit map memory 7. After all objects of a band have been rastered by the raster generator 5 and written into the bit map memory 7, the content of the bit map memory 7 is forwarded as control signal values to the recorder 8 and exposed thereat.
As a rule, it is not only one but a plurality of printed pages at once that are printed with a printing plate, these being arranged such that the area of the printing plate is used well and such that, after folding and cutting the printed paper sheet, the printed pages yield a brochure, a leaflet or the like. For that purpose, the printed pages that are to be printed on the paper sheet in the same printing event are combined in a signature. The arrangement of the printed pages in a signature is referred to as an imposition pattern. As an example, FIGS. 4a and 4b show the imposition patterns for a brochure with 16 printed pages. FIG. 4a shows a printing plate 16 on which a signature 17 is arranged. The signature 17 unites all elements to be printed, i.e. the printed pages 18 and auxiliary elements such as register marks 19, fold and cut marks 20 and print control strips 21. These auxiliary elements serve for quality control during printing and for simplifying the further-processing (folding, cutting, binding). The numbers in the printed pages 18 in the imposition pattern identify which page of the brochure is printed at which location of the signature. Numbers that are upside down identify pages that are printed upside down. FIG. 4a shows the pattern that is printed on the recto of the paper sheet (obverse) and FIG. 4b shows the pattern that is printed on the verso of the same paper sheet (reverse). With the imposition pattern of FIGS. 4a and 4b, the pages are arranged continuously in the brochure after the printing of both sides of the paper sheet and after the folding and cutting.
In the prior art, there are two essential methods for assembling pages that are present as PostScript data to form a signature, manual assembly and electronic assembly of the PostScript data. In manual assembly, the PostScript data of all pages are first interpreted in a RIP, and the color separation films of the pages are exposed on a recorder, as shown in FIG. 2. 64 color separation films thus arise in the example of the brochure with 16 pages (16 pages.times.4 inks). Eight printing plates each having Eight pages are required for the printing (4 respective inks for the obverse and the reverse). Given manual assembly of the 8 signatures, 8 color separation films of the pages per signature must be glued onto a transparent film having the size of the signature according to the arrangement of the imposition pattern, for example the cyan films of pages 1,4,5,8,9,12,13,16 according to the imposition pattern of FIG. 4a. The films of the other inks are likewise respectively glued onto a large assembly film according to the same pattern. One proceeds accordingly for the color separation films of pages 2,3,6,7,10,11,14,15 but according to the imposition pattern of FIG. 4b. The printing plates are then produced by contact exposure with the assembled films in a photographic process. Manual assembly work must be very carefully and exactly carried out since the signatures of the individual inks must be congruent so that no color fringes occur at the image or text edges in the final print and so that the sharpness of the printed images is not deteriorated. It is obvious that manual assembly of the signatures is extremely work-intensive, time-consuming and susceptible to error as well.
The production of a PostScript description of the entire signature is standard as an electronic assembly method for signatures in the prior art. For that purpose, the PostScript data of all pages are collected in a preliminary process on a computer (server), and, when the are completely present, are linked with PostScript data for the register marks, fold/cut mark and print control strips to form an extensive file of PostScript data for the entire signature. A respective PostScript file is usually generated for each ink and for each side of the paper sheet. In the example of the 16-page brochure, 8 PostScript files thus arise for the 8 signatures (respectively 4 inks for the obverse and the reverse). These PostScript files are then interpreted in the RIP, screened and exposed in a large-format recorder on films having the size of the signatures or directly on printing plates.
It is also standard to mix both methods, for example the PostScript assembly of respectively half of a signature and the manual assembly of the two halves to form an entire signature. On the one hand, a recorder (expensive) with a very large exposure format is not required; on the other hand, however, the manual assembly work is greatly simplified.
PostScript assembly of the signatures also has disadvantages. First, the PostScript data for a signature can be extremely extensive and complex, so that a very high-performance and, thus, expensive computer is required in the RIP for the interpretation. Since the individual printed pages are often produced by used programs from different manufacturers (text processing, graphic design and image processing programs), it can occur that the PostScript data of some pages cannot be correctly processed by the interpreter in the RIP or that the RIP even freezes up during the exposing. This is the case when the manufacturers of the user programs have not exactly adhered to the rules of the PostScript page description language. This is more critical in exposing signatures than when exposing single pages since the signature exposing can only be started when all pages are finished. This, however, often occurs only shortly before that start of printing, so that there is no more time to search for the error.
There would also fundamentally be the possibility of having the individual pages interpreted and screened by the RIP and by not immediately forwarding the bit map data thereby generated to the recorder for exposure but, for example, to intermediately store them on a disk storage. The bit map data of all pages could then be operated in a computer (server) according to the imposition pattern to form a bit map dataset for the entire signature and then be subsequently exposed. Such a system is disclosed in published application DE 40 26 321 A1, whereby images are screened and stored as compressed bit map data. This solution, however, is not practical for higher exposure resolutions as required for the recording of printer's copies since the memory for the signature bit map becomes extremely large and expensive. A memory requirement of 3109 Mbytes per signature derives for a printing plate having the size 70 cm.times.100 cm and a resolution of 2666 pixels/cm horizontally (6772 dpi; dpi=dots per inch) and 1333 lines/cm vertically (3383 dpi). A storage space of 24876 Mbytes is then required for the 8 signatures of the 16-page brochure. Hard disks are also eliminated as a storage medium since they cannot read out the bit map data with the required recorder data rate of 100 to 200 Mbits/s.
Due to the high memory requirement for the finished bit map of a signature, the bit map of a signature cannot be intermediately stored in the previous procedure for the assembly of PostScript data. When the same signature is to be exposed again, for example because the film exposed first or the printing plate was damaged, the entire processing sequence from the interpretation of the Post Script data up to the exposure must be run through again. This costs additional time and occupies the RIP that could already process a new signature during this time. For the same reason, the additional exposure of the signature on a proof output device in the previous procedure again requires the entire run of the PostScript data through the RIP and therefore costs unnecessary time. This is a further disadvantage of the assembly of PostScript data in the prior art.